Thin film photovoltaic (PV) solar cells are one class of energy source devices which harness a renewable source of energy in the form of light that is converted into useful electrical energy which may be used for numerous applications. Thin film solar cells are multi-layered semiconductor structures formed by depositing various thin layers and films of semiconductor and other materials on a substrate. These solar cells may be made into light-weight flexible sheets in some forms comprised of a plurality of individual electrically interconnected cells. The attributes of light weight and flexibility gives thin film solar cells broad potential applicability as an electric power source for use in portable electronics, aerospace, and residential and commercial buildings where they can be incorporated into various architectural features such as roof shingles, facades, and skylights.
Thin film solar cell semiconductor packages generally include a conductive back contact or electrode formed on a rear glass or polymer substrate and a conductive front contact or electrode formed above the back electrode. Front electrodes have been made for example of light transmittance conductive oxide (“TCO”) film materials. A light-absorbing active or absorber layer (“ABS”) is interspersed between front and back electrodes which absorbs the solar radiation photons and excites electrons to produce an electric current thereby chemically converting solar energy into electrical energy.
Processes used to form absorber layers made of chalcogenide materials such as copper indium diselenide species (CIS), copper indium gallium diselenide species or Cu(In,Ga)Se2 (“CIGS”) or Cu(In,Ga)(Se, S)2 (“CIGSS”) involve a furnace-based selenization/sulfurization process. Generally, base materials such as copper, indium, and gallium (for CIGS or CIGGS absorber layers) are sputtered or otherwise deposited on the back electrode of the solar cell substrate. The substrate is then loaded into a furnace where a carrier gas containing selenide is introduced followed by introducing gas containing sulfide, all of which is coupled with heating.
In additional processes used in forming the thin film solar cell, buffer layers made of cadmium sulfide (CdS) are formed on the absorber layer commonly by a chemical bath deposition (CBD) process wherein the entire substrate is immersed in an electrolytic chemical bath.
The foregoing selenization/sulfurization and CBD processes create unwanted carryover and formation of chemical compound deposits on solar cell surfaces other than the intended target areas. Accordingly, it is generally desirable after forming the foregoing absorber and buffer layers to etch and clean the backside surface of the rear glass substrate to remove any chemical compound debris or deposits that may have adhered to and contaminated this surface to avoid potential performance degradation of the solar cell or appearance defects. The rear substrate backside etching/cleaning operations use a combination of chemical etching, brushing, and water. Chalcogenide absorber layer films formed on the opposite side of the rear glass substrate, however, are susceptible to peeling and other forms of damage if exposed to moisture, water, and etching solutions. This can cause appearance defects and more significantly adversely affects the reliability of the solar cell. Therefore, it is useful to protect the absorber layer from exposure to water and moisture when cleaning the rear substrate.
All drawings are schematic and are not drawn to scale.